The pills weren't poison. They were medication for his aneurysm which is only deadly to people without any aneurysm, and both bottles had the same type of pills, so he was safe no matter what, and the other person was dead no matter what.

I mean, why give the victim 50/50 odds when you could make it dead certain?

The cabbie thought he WAS certain. He would make the first move and he was so confident in his ability to read people, he thought he could get them to pick the poison one. As he said "It's NOT chance, it's chess."

Pretty sure no medication works that way. That's a ridiculously potent on-target effect which can kill a person in a short time but only treat the patient. With no side-effects.

I suppose you could conjecture that he's been built-up to a massive dose (of what - beta blockers? Not likely to kill that quickly) or that the Sherlock universe has magical drugs, but there's nothing to support that.

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